1. Field of the Invention
The present invention relates to an apparatus for measuring the pulse transmission spectrum of elastically scattered X-ray quantities.
2. Description of the Related Art
An apparatus of the above-described type has been disclosed in DE-A-42 22 227 and EP-B1-0 496 454. The teachings of these references introduce examination or detection systems for large objects which are based on the detection of the elastic scattering of x-rays. The elastically scattered x-ray radiation exhibits in its energy spectrum characteristic structures which makes it possible to make conclusions concerning the type of the scattering material. For example, it is possible to identify materials such as explosives or drugs in containers such as travel luggage, packages, letters, or the like (H. Strecker; Automatische Gepxc3xa4ckkontrolle mit Rxc3x6ntgenstreustrahlung; [Automatic luggage control using x-ray radiation] Physik in unserer Zeit 30, 31-34 (1999).
The principal difference of the two scattered ray systems described above resides in the use of different geometric configurations which are utilized for radiating the examined object. In DE-A-42 22 227, a cone-shaped ray bundle is used for radiation, as it is illustrated in FIG. 1 with an exaggerated aperture angle. The primary ray bundle is masked from the x-ray lobe which leaves the x-ray focus by an appropriate annular diaphragm. The diameter of the primary ray cone in the object space initially determines the horizontal spatial local resolution. The resolution is in the order of magnitude of several cm and can be varied only insignificantly by the choice of the primary cone aperture angle and the distance of the primary collimator diaphragm from the focus because these two variables are significantly limited by other system parameters.
Examined objects which are located in the object space partially scatter the primary x-ray light (see the right side of FIG. 1). Portions of this scattered light are directed to a circular ring-shaped segmented detector through a secondary collimator which is essentially composed of a circular ring-shaped imaging or focusing diaphragm system and a detector diaphragm system. By using several slot systems arranged next to each other it is possible to divide the entire object space vertically into several partially overlapping image layers. For example, FIG. 2 shows how two image layers located one above the other can be focused through an imaging slot onto two detector elements.
Horizontal scanning of the examined object takes place by a meander-like displacement of the x-ray and detector system and the examined object with a timed detection of the x-ray signals in the detector.
The spatial resolution of the examined partial volumes (voxels) of the object space depends
in x-direction on the relative displacement speed, the integration time intervals and the diameter of the primary x-ray cone,
in y-direction on-the diameter of the primary x-ray cone,
in z-direction on the slot width of all involved slot diaphragms.
A first attempt for reducing the resolution can be found in DE-A-44 45 876 in which semicircular diaphragm structures and a semicircular structured detector are used. The left and right diaphragm system are arranged displaced relative to each other in such a way that the resulting imaging layers from the left and right system halves are vertically nested into one another and overlap each other partially.
The resolution in the x-direction now only depends on the relative displacement speed and the integration time intervals.
In contrast to DE-A-42 22 227, EP-B1 0496454 utilizes a pin ray for radiating the examined object. A principal sketch for this system is shown in FIG. 3. The needle ray is masked from the x-ray lobe leaving the x-ray focus by means of a small pin diaphragm having a diameter of about 1 mm. Since the divergence of the pin ray is very small, its diameter in the object space is also only about 1 mm.
Consequently, the lateral resolution of the partial volumes (voxels) of the pin ray system is
very fine in the y-direction
adjustable in the x-direction by the relative displacement speed and the integration time intervals,
dependent in the z-direction on the slot width of all involved slot diaphragms.
The necessity of adjusting the local resolution to the examined object is a consequence of the type of examination to be performed. For example, the scattered ray geometry used for examining containers for different materials, such as travel luggage with explosives or drugs in minimum quantities of several 100 g is significantly different from the examination of briefcases, packages and letters for significantly smaller quantities of explosives or drugs in the range of a few 10 g. Consequently, it is desirable to have a scattered x-ray system available which is capable simultaneously to detect large and small quantities of substances. It is understood that an easy and quick manipulation of the apparatus should be possible.
Therefore, it is the primary object of the present invention to improve the apparatus described above in such a way that it can be adapted to the respective object to be examined and its contents. The apparatus should make it possible, for example, after a coarse scan of a container, such as a piece of luggage or the like, in suspicious cases to once again locally examine the critical areas with a finer resolution or, if there is already an initial suspicion, to carry out a full scan with high local resolution, i.e., with high sensitivity for small quantities.
In accordance with the present invention, the arrangement for measuring the pulse transmission spectrum of elastically scattered x-ray quantities of the above-described type includes
a polychromatic x-ray emitter;
a primary diaphragm arrangement arranged between the x-ray emitter and an examination area through which the x-ray radiation is conducted, wherein the primary diaphragm arrangement serves to mask a primary ray bundle or pin ray bundle extending through the examination area on the surface of a cone;
a detector arrangement composed of several detector elements concentrically surrounding the detector center point for determining the x-ray quantities scattered in the examination area;
a secondary diaphragm arrangement arranged between the examination area and the detector arrangement, wherein the secondary diaphragm arrangement includes at least one imaging slot for imaging the scattered x-ray radiation on the detector arrangement;
a device for the relative displacement between an examination object placed in the examination area and the examination arrangement for scanning the examination object in a scanning direction; and
means for processing the measured signals, wherein the detector arrangement is divided by means of separating areas arranged perpendicularly of the scanning direction into a least two detector areas having essentially the same width, and wherein each detector area has a number of partial ring-shaped detector elements, and each detector element is provided with means for signal processing.
According to the present invention, the arrangement described above further includes
a) at least one diaphragm displaceable in the y-direction for switching between fine and coarse local resolution of the examined partial volumes of the examination object, wherein the diaphragm narrows the effective ray width transversely of the scanning direction, and
b) a diaphragm system displaceable in the x-direction for limiting the z-extension of the partial volumes, wherein the diaphragm system includes at least two circular ring diaphragms which are arranged one above the other and are identical with respect to their circular ring structure, wherein the circular ring diaphragms are arranged at a relative offset for reducing the effective imaging slot width or the detector slot width.
Accordingly, the present invention provides an arrangement in which the local resolution can be quickly switched by means of slidable diaphragms. The mechanical switching by means of diaphragms is provided for the y-direction and the z-direction of the local resolution.
The improvement of the local resolution xcex94x=V*xcex94t in the x-direction takes place by a reduction of the relative speed V or/and a shortening of the integration time xcex94t. FIG. 4a shows the resulting cross-sections of a voxel sequence for the case of a cone-shaped ray system with semicircular ring detector elements with a long measuring time xcex94tg and/or a high relative speed Vg, while FIG. 4b shows the sequence of the voxel cross-sections for short measuring times xcex94tk and/or low speeds Vk.
The improvement of the local resolution in the y-direction takes place by a displaceable diaphragm or pair of diaphragms extending into the x-ray bundle. It is almost entirely insignificant whether the primary ray cone is already limited by an additional diaphragm pair (y-diaphragm pair) in or below the primary collimator in its diameter in the y-direction, or whether the width of the scattered light bundle is narrowed immediately above the secondary collimator, in the secondary collimator or underneath the secondary collimator directly in front of the detector through a y-diaphragm pair. Preferred embodiments are shown in FIG. 5a for the y-diaphragm pair on the primary side and in FIG. 5b for the y-diaphragm pair on the secondary side. The result of the narrowing of the cross-section of the primary or secondary x-ray bundle is shown in FIG. 4c. FIG. 4c shows the row of narrowed voxel cross-sections arranged successively in the y-direction. The use of the plate-shaped pair of y-diaphragms requires a relatively long opposite movement of the two components, as well as a certain space requirement outside of the circular diaphragms, if the path of the rays is not to be narrowed; therefore, the partial y-diaphragm shown in FIGS. 5c and 5d is preferred.
The slidable diaphragms or diaphragm systems have the purpose to narrow or limit the z-extension of the voxels. This applies equally to voxels resulting from a cone-shaped ray and to the voxels of a pin ray system.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.